Railroad tank cars are required to be equipped with a pressure relief device which will open in the event of a major pressure surge in the car. The purpose of the relief device is to open and vent major pressure surges in a controlled manner rather than risk damage to the tank car body or a catastrophic release of pressure. Major pressure surges, also sometimes known as hydraulic hammer, can be caused by sudden, abnormal shifting of the liquid contents of a car due to improper car handling, emergency braking or the like. The car body and relief device are designed to withstand minor pressure surges caused by normal car handling and movement. Shifting or sloshing of the liquid contents occurs because the tank cars are not permitted to be filled fully. Thus, there is some open space at the top of the car body, allowing for movement of the liquid within the car, which can compress the air (or other gas) at the top of the car and create a pressure surge.
A safety vent for a railroad tank car mounted about an opening on the top of the car is taught by Burian in U.S. Pat. No. 5,785,078 assigned to the present assignee. The housing body of the safety vent defines a cavity which contains a surge protector and a rupture disk. The rupture disk is designed to open at known pressure and allows controlled venting of the pressure in the car. A securement cap threaded into the cavity entraps the rupture disk in a seat formed in the surge protector. Removal of the securement cap provides quick, easy access to the rupture disk and surge protector for inspection and replacement without removing the safety vent housing body.
The present invention represents a refinement in the development of manway cover assembly by mounting a safety vent of the type taught in U.S. Pat. No. 5,785,078 on the top of a manway cover housing while providing means to protect the safety vent and allowing optimal operation of the safety vent.